The closest known approaches to the web coating process of this invention are described in Koslow U.S. Pat. Nos. 5,019,311; 5,147,722; 5,189,092; 5,249,948; and 5,331,037, their parent applications, their corresponding foreign patent applications and patents, and the references cited therein. See also co-pending patent application Ser. No. 08/813,055, filed concurrently herewith by the present inventors and entitled "Continuous Solid State Web Coating Apparatus and Webs Produced Thereby," now U.S. Pat. No. 5,792,513. The closest known prior art to the flow-through capacitors of this invention are U.S. Pat. Nos. 5,192,432; 5,360,540; 5,415,768 inter alia of Marc D. Andelman.
The above-mentioned Koslow patents disclose processes for the production of composite materials which are characterized by primary particles interconnected by a binder material. Some of these processes require high pressure and shear or extrusion through a die with carefully controlled back pressure. These prior art processes are extremely useful in producing a wide variety of articles including extruded solid forms such as activated carbon filters.
The above-mentioned Andelman patents disclose capacitors designed to receive fluids therethrough for various applications such as chromatography and fluid purification. These capacitors operate by removing ionized particles and solutes by electrical attraction to the charged plates. In one embodiment, Andelman proposes a wound cylindrical capacitor having radial flow between the inside and the outside of the cylinder. Such an arrangement has the advantage of efficiency. However, in order to obtain such a construction, six or eight separate and individual layers must be wound around a central core. In accordance with the present invention, a cylindrical capacitor of somewhat similar configuration is obtained by use of a layered composite electrode produced by the method of this invention.
It would be desirable to impregnate, cover, or otherwise treat, a relatively fragile, fluid permeable, dielectric base material web with an electrically conductive material that has a high surface area and is also porous to fluid flow. One example would be a nonwoven or plastic web coated with carbon particles and binder particles on at least one side. Two such coated webs could be wound together as, for example, in the known method of manufacturing a metallized film capacitor. This would produce a capacitor capable of functioning in essentially the same manner as the prior art flow-through capacitors but would be much easier to manufacture. However, the fragile nature of the underlying base material would make it impractical to employ the known prior art techniques which require high pressure and shear. Furthermore, the most desirable binder materials are in the form of extremely finely divided particulate material which is difficult to employ because it is non-flowable due to the high innate cohesion between the particles created by electrostatic and van der Waal forces.
In the prior art referred to above, the powdered active material is caused to bind to the substrate by means of a thermoplastic material with which it is intimately mixed. However, the pressures and temperatures involved would not permit their application to fragile substrates such as the webs described herein. Accordingly, it is a primary object of the present invention to provide a method for continuously coating a relatively fragile web with a particulate conductive material and a very finely divided particulate thermoplastic binder. Another object is to produce a flow-through capacitor from such a web. Other objects, features, and advantages will become apparent from the following description and appended claims.